G-protein-coupled receptors constitute the largest family of cell surface receptor proteins. There are three major families of GPCRs, Gs-, Gi-, and Gq-coupled receptors. Upon activation, different GPCRs stimulate a number of signal transduction pathways. For example, Gs-coupled receptor increases while Gi-coupled receptor decreases cAMP production. Therefore, these two different GPCRs can activate or inhibit the cAMP-response element. On the other hand, Gq-coupled receptor increases intracellular calcium concentration and activates the multiple-response element.
Many human diseases are associated with the dysfunction of GPCRs. GPCRs are the most attractive therapeutic targets in the pharmaceutical industry. A large number of technologies have been developed to detect GPCRs activity. However, these technologies can only be used with certain types of GPCRs. There are no existing assay systems that can be employed to universally detect ligand-binding activities of all types of GPCRs.
Therefore, a need exists for real universal assay system to identify ligands or modulators of various GPCRs from different biological samples. The present invention fulfills this and other needs.